Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G - 1 to allow up to 4G initrd

[Li Zhijian]

On 11/9/2018 3:20 PM, Ingo Molnar wrote:
> * Li Zhijian <lizhijian@xxxxxxxxxxxxxx> wrote:
>
> > > If the kernel initrd creation process creates an initrd which
> > > is larger than 2GB and also claims that it can't be placed
> > > with any part of it above 2GB, then that sounds like a bug
> > > in the initrd creation process...
> > Exactly, it's a real problem.
> >
> > Add x86 maintainers and LKML:
> >
> > The background is that QEMU want to support up to 4G initrd. but linux header (
> > initrd_addr_max field) only allow 2G-1.
> > Is one of the below approaches reasonable:
> > 1) change initrd_addr_max to 4G-1 directly simply(arch/x86/boot/header.S)?
> > 2) lie QEMU bootloader the initrd_addr_max is 4G-1 even though header said 2G-1
> > 3) any else
> A 10 years old comment from hpa says:
>
>    initrd_addr_max: .long 0x7fffffff
>                                          # (Header version 0x0203 or later)
>                                          # The highest safe address for
>                                          # the contents of an initrd
>                                          # The current kernel allows up to 4 GB,
>                                          # but leave it at 2 GB to avoid
>                                          # possible bootloader bugs.
>
> To avoid the potential of bugs lurking in dozens of major and hundreds of
> minor iterations of various Linux bootloaders I'd prefer a real solution
> and extend it - because if there's a 2GB initrd for some weird reason
> today there might be a 4GB one in two years.

thank a lots. that's amazing.


> The real solution would be to:
>
>   - Extend the boot protocol with a 64-bit field, named initrd_addr64_max
>     or such.
>   - We don't change the old field - but if the new field is set by new
>     kernels then new bootloaders can use that as a new initrd_addr64_max
>     value. (or reject to load the kernel if the address is too high.)
>
>   - The kernel build should also emit a warning when building larger than
>     2GB initrds, with a list of bootloaders that support the new protocol.

Actually i just knew QEMU(Seabios + optionrom(linuxboot_dma.bin)) can support ~4GB initrd so far.

i just drafted at patch to add this field. could you have a look.
another patch which is to document initrd_addr64_max is ongoing.

commit db463ac9c1975f115d1ce2acb82d530c2b63b888
Author: Li Zhijian <lizhijian@xxxxxxxxxxxxxx>
Date:   Fri Nov 9 17:24:14 2018 +0800

    x86: Add header field initrd_addr64_max
    
    Years ago, kernel had support load ~4GB initrd. But for some weird reasons (
    avoid possible bootloader bugs), it only allow leave initrd under 2GB address
    space(see initrd_addr_max fild at arch/x86/boot/header.S).
    
    So modern bootloaders have not chance to load >=2G initrd previously.
    
    To avoid the potential of bugs lurking in dozens of major and hundreds of
    minor iterations of various Linux bootloaders. Ingo suggests to add a new field
    initrd_addr64_max. If bootloader believes that it can load initrd to >=2G
    address space, it can use initrd_addr64_max as the maximum loading address in
    stead of the old field initrd_addr_max.

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 4c881c8..5fc3ebe 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -300,7 +300,7 @@ _start:
        # Part 2 of the header, from the old setup.S
 
                .ascii  "HdrS"          # header signature
-               .word   0x020e          # header version number (>= 0x0105)
+               .word   0x020f          # header version number (>= 0x0105)
                                        # or else old loadlin-1.5 will fail)
                .globl realmode_swtch
 realmode_swtch:        .word   0, 0            # default_switch, SETUPSEG
@@ -562,6 +562,12 @@ acpi_rsdp_addr:            .quad 0                 # 64-bit physical pointer to the
                                                # ACPI RSDP table, added with
                                                # version 2.14
 
+#ifdef CONFIG_INITRD_SIZE_4GB
+initrd_addr64_max:     .quad 0xffffffff        # allow ~4G initrd since 2.15
+#else
+initrd_addr64_max:     .quad 0
+#endif
+
 # End of setup header #####################################################
 
        .section ".entrytext", "ax"
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 22f89d0..b86013d 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -90,6 +90,7 @@ struct setup_header {
        __u32   init_size;
        __u32   handover_offset;
        __u64   acpi_rsdp_addr;
+       __u64   initrd_addr64_max;
 } __attribute__((packed));
 
 struct sys_desc_table {
diff --git a/init/Kconfig b/init/Kconfig
index a4112e9..611d4af 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1080,6 +1080,14 @@ config BLK_DEV_INITRD
 
          If unsure say Y.
 
+config INITRD_SIZE_4GB
+       bool "4G size initrd support"
+       depends on (X86 || X86_64)
+       help
+         This option enables support ~4GB initrd.
+
+         if unsure say N.
+
 if BLK_DEV_INITRD
 
 source "usr/Kconfig"

Thanks
Zhijian




> Or something along those lines.
>
> Thanks,
>
> 	Ingo